1. Field of the Invention
The present invention relates to a method and an apparatus of electromagnetic measurement. More specifically, the present invention relates to a method and an apparatus of electromagnetic measurement, for measuring electromagnetic waves emitted from an electronic device, constituent elements thereof and peripheral devices thereof, mainly, electromagnetic interference in an electromagnetic near field.
2. Description of the Related Art
In order to prevent that unnecessary radiation from an electronic device interferes with another device and exerts harmful influences on a human body, recently, regulations are established in many countries. At overseas, for example, there are established standards by FCC (Federal Communications Commission), CISPR (Comittie International Special des Perturbations Radioelectrique), VDE (Verband Deutscher Elektrotechniker) and the like. Also in Japan, there is established voluntary restraint standards by VCCI (Voluntary Control Council for Interference by Information Technology).
In general, regulations for unnecessary radiation, that is, electromagnetic interference are established for respective frequencies in a wide range of 30 MHz to 1 GHz. This range includes an electromagnetic field at an extremely low frequency of 50 Hz to 60 Hz on a power transmission line, and radio waves from a very low frequency in an electromagnetic cooker to a very high frequency in a TV set, a microwave oven and a portable telephone.
Electromagnetic is a wave that an electric field and a magnetic field vibrate in a pair to transmit electromagnetic energy to a space. Measurement for electromagnetic interference is performed as follows: an electric field intensity or a magnetic field intensity is measured at a position spaced away from an electronic device, serving as a source of electromagnetic, by a predetermined distance. In order to block extraneous noise, a special environment and a special apparatus are used for this measurement. Consequently, the measurement requires expert knowledge, and takes much time.
In order to solve the aforementioned disadvantages, an electromagnetic measurement apparatus for measuring an intensity of near electromagnetic emitted from various circuits such as a printed circuit board in an electronic device is used for reducing time and cost for measurement. For example, there is known that such an apparatus uses an electric field sensor or a magnetic field sensor to measure an electric field intensity or a magnetic field intensity near an electronic device, prepares a frequency spectrum based on a maximum value of the measured magnetic field intensity and prepares electromagnetic field intensity distribution at a high frequency based on the frequency spectrum of the maximum value to thereby search for an emission noise source, analyze an emission noise generation mechanism and confirm a countermeasure effect (see, e.g., JP2002-372558A). There is also known the following technique: an antenna array including a plurality of antennas is used for receiving electromagnetic emitted from a device under test by one of the antennas arranged at a distance from the device under test, and detecting an electromagnetic emission pattern from the device under test based on the received information to thereby reduce time for measurement (see, e.g., JP2004-77336A).
In any of the aforementioned techniques, in order to identify a cause and a site of electromagnetic interference from an electronic device, it is necessary to scan a whole range of an electromagnetic near field in the device under test by use of one or a limited number of receivers with directivity. As disclosed in JP2002-372558A and JP2004-77336A, the receiver scans a position at a predetermined distance from the electromagnetic near field in the device under test.
There is also known a method of obtaining distribution of currents passing through a device under test from distribution of magnetic fields measured by a magnetic field sensor at a position spaced away from the device under test by an optional distance, and obtaining an electric field intensity at the optional distance based on the current distribution (see, e.g., JP2003-279611A).
There is also known the following technique. In a case of measuring a device under test having surface irregularities, a distance sensor for measuring a distance between an electromagnetic field sensor and the device under test is used together with the electromagnetic field sensor. An electromagnetic field intensity measured by the electromagnetic field sensor is subjected to distance correction in accordance with a distance between the device under test and the electromagnetic field sensor (see, e.g., JP2000-230954A).
Next, description will be given of measurement of electromagnetic interference with reference to FIGS. 9A to 9C. Herein, an antenna “b” disclosed in JP2004-77336A has a received band in a device under test “a” widened in accordance with a measurement distance. For example, FIG. 9A illustrates a received band AN in a case of a measurement distance DS between the antenna “b” and the device under test “a”, and FIG. 9B illustrates a received band AW in a case of a measurement distance DL longer than the measurement distance DS.
An electromagnetic intensity is inversely proportional to a square root of a distance. Therefore, as illustrated in FIG. 9A, if the measurement distance DS is short, the antenna “b” readily receives electromagnetic and a measurement resolution can be enhanced. However, a received band becomes narrow as shown by the received band AN. If the received band becomes narrow, a number N of received bands in a whole range of a measurement region is increased by n (n=48 in a schematic diagram of FIG. 9C), so that it takes much time t to perform measurement, that is, t=Tn. When the received band is widened by the measurement distance DL within a range of an electromagnetic near field as illustrated in FIG. 9B, the received band is widened as shown by the received band AW, so that the number N of received bands is decreased by m. As a result, the measurement time t is reduced without effort, that is, t=Tm. However, since an electromagnetic intensity to be received is lowered and a resolution for measurement is also lowered, measurement with high accuracy cannot be performed.
At present that measurement with high accuracy is indispensable in order to offer a product without a problem about electromagnetic interference, if electromagnetic interference is measured upon designing and manufacturing a product, a whole electromagnetic emission region of the device under test “a”, including a region having no electromagnetic interference, must be scanned with a high resolution in such a manner that a measurement distance is set at the measurement distance DS and a received band is set at the received band AN. This results in prolonged time for measurement and increased cost for a product.
Moreover, an electromagnetic measurement apparatus does not receive radiation from a device under test, but receives extraneous noise from a farfield. In order to avoid such a situation, it is necessary to place the device under test and the electromagnetic measurement apparatus in a large-size shield room or a large-size anechoic room. This results in increased cost for measurement, that is, increased cost for a product.
It is an object of the present invention to provide a method and an apparatus of electromagnetic measurement, that can perform measurement with high accuracy in a short time and can be realized in a compact facility.
If a conventional electromagnetic measurement apparatus estimates a current source which is a source of unnecessary radiation from a device under test, the following problem arises. In a method of estimating a current source from a single electromagnetic field, a degree of accuracy for estimating a current at each measurement point on the device under test varies due to variety such as a mechanism, a structure and a material of each measurement point. Consequently, uniform evaluation cannot be performed in an area to be measured.
Further, if an electromagnetic field sensor moves within a plane spaced away from a device under test having surface irregularities by a predetermined distance in measurement of electromagnetic field distributions in electromagnetic containing unnecessary radiation from the device under test, distances between the device under test and the electromagnetic field sensor are different from each other at the respective measurement points. Therefore, even when electromagnetic field intensities are equal to each other in the measurement plane, if the distances between the device under test and the electromagnetic field sensor are different from each other, actual electromagnetic field intensities by electromagnetic emitted from the device under test are different from each other.
In order to solve the aforementioned disadvantage, there are a method of allowing an electromagnetic field sensor to move along irregularities on a surface of a device under test to perform measurement, and a method of measuring a distance between an electromagnetic sensor and a measurement point by a distance sensor, and subjecting an intensity of an electromagnetic field by electromagnetic emitted toward the measurement point to distance correction. However, these methods require much time. Further, since this measurement does not respond to a mechanism, a structure and a material of a device under test, a current cannot be measured correctly. Consequently, it is difficult to ensure accuracy of measurement.
Therefore, it takes much time to accurately detect unnecessary radiation from a device under test having a complicated surface structure. Hence, there arises a problem that it takes much time and cost to correctly grasp a failure of a device under test, which is a cause of unnecessary radiation.
It is another object of the present invention to provide a method and an apparatus of electromagnetic measurement, that can accurately detect unnecessary radiation from a device under test having a complicated surface structure in a short time, and can correctly grasp a failure of a device under test, which is a cause of unnecessary radiation, at a low cost.